Field of the Invention
The present invention generally relates to optical fiber cables (shortly, optical cables). More specifically, the present invention relates to an optical cable suitable for air-blown installations, capable of housing a high optical fiber count and featuring flame-retardant properties.
Overview of the Related Art
In the last years, it has become important to link end users to existing optical telecommunication networks. Particularly in urban areas, linking the end users to an optical telecommunications network requires the availability of compact optical cables containing a high optical fiber count, suitable to be laid down in already existing cable routing conduits, possibly exploiting efficient deployment techniques such as the air-blowing installation (as for example specified in standard IEC 60794-5-10).
Another important requirement for the optical cables is a good behavior in presence of fire (as specified for example in standard IEC 60332-3C), so as to prevent fire and smoke propagation in houses or, more generally, in premises where persons reside.
A known optical cable for air-blown installation technique, as for example disclosed in WO 2005/040882, has a Multi Loose Tube (MLT) design and a jacket made of a blend of Low Smoke Zero Halogen material (LSZH, also known as LSOH or LSOH or LSFH or OHLS material, i.e. a material typically used for cable jacketing in the wire and cable industry, composed of thermoplastic or thermoset compounds that emit limited smoke and no halogen when exposed to high sources of heat, e.g. flame). The MLT design or configuration comprises a central strength member and a number of tubes (buffer tubes) arranged around the central strength member and loosely accommodating optical fibers.
The MLT configuration, thanks to its symmetrical structure, is considered well fitting air-blown installation.
LSZH blends used for the optical cable jacket generally comprise a polymeric material charged with a flame-retardant inorganic filler. These blends have a LOI (Limiting Oxygen Index)—a parameter indicating the minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer; the higher the LOT the better the flame-retardant behavior—typically of about 45÷50%, together with a medium hardness, typically of about 50÷55 Shore D, because of the presence of the inorganic filler/s. As a consequence, polymer blends with high LOT generally has a limited resistance to friction.
Another known design or configuration for optical cables is the Central Loose Tube (CLT) one, as disclosed for example in WO 2005/040883 and WO 2010/105657. The CLT design comprises a number of tubes loosely containing optical fibers to form the so-called optical micro-modules, and a plastic jacket surrounding said tubes, with strength members embedded in the jacket. The presence of strength members, typically two or in two groups diametrically opposed, provides the cable with a preferential bending plane making its structure asymmetric.
WO 03/046074 describes a telecommunication cable comprising a sheath housing at least one transmission element, said sheath comprising a polymeric composition. The addition of inorganic filler to the polymeric composition, particularly in amounts equal to or higher than about 50% by weight with respect to the total weight of the polymeric composition, may confer advantageous flame retardant properties to the polymeric composition. The cable comprises: a plurality of optical sub-units in the form of tubes, each sub-unit encircling a plurality of optical fibers; a polymeric sheath (made from a polymeric composition as above defined) containing the optical sub-unit; a couple of longitudinal reinforcing elements embedded in the polymeric sheath.
WO 2011/035814 relates to optical cables for communications including at least one micro-module. The problem faced in this application is that of reducing the external diameter of the micro-modules for packing them in a number as high as possible within the optical cable sheath. This allows obtaining very compact micro-modules and accordingly optical cables with increased optical fiber density. The optical cable comprises a number of micro-modules forming an optical core, and an outer sheath. Two opposite reinforcing longitudinal elements are preferably arranged in the thickness of the outer sheath. An optical cable may comprise 24, 36 or 60 micro-modules, each comprising 12 optical fibers. The retaining element is generally made of a polymeric material, in particular a thermoplastic material, optionally charged with mineral fillers.